The invention is useful in code division multiple access digital data transmission systems and other digital data transmission systems using carriers to eliminate or substantially reduce narrow band noise power.
In a paper by Jeffrey A. Young and James Lehnert, entitled xe2x80x9cAnalysis of DFT-based Frequency Excision Algorithms for Direct-Sequence Spread-Spectrum Communicationsxe2x80x9d published in August 1998 in IEEE Transactions on Communications, Vol. 46, No. 8, p. 1076 (hereafter the Young paper), the authors describe a discrete Fourier transform type frequency excision algorithm to eliminate narrow band noise from direct-sequence spread-spectrum modulation. The authors note that processing gain limits the interference rejection capability of unaided direct-sequence spread-spectrum modulation. The prior art contains numerous narrow band interference rejection techniques called frequency excision algorithms to extend the interference rejection capability of spread spectrum systems beyond the processing gain limits. This is an important ability to overcome severe interference situations when strong narrow band spurious signals are received along the spread spectrum signal.
This frequency excision capability is highly desirable in, for example, the new digital data delivery systems for delivering high bandwidth telephone service, video on demand and high speed internet access to subscribers on cable TV systems via the hybrid fiber-coax cable plant. Frequency excision also allows transmitted power levels to be reduced.
The prior art adaptive notch filtering techniques described in references 1-3 of the Young paper are noted to be cumbersome and adapt slowly to the frequency of the interfering signal. Adaptive A/D conversion described in reference 4 of the Young paper works for a single CW interference source but cannot be easily generalized to multiple interference sources. Transform domain signal processing using SAW filters described in references 5-11 of the Young filter promises wide communication bandwidths and rapid adaptation to changing interference. However, a drawback to using SAW filters is that they limit the linear dynamic range and, hence, limit the immunity to multiple interferers. A class of excision algorithms called adaptive digital filtering (ADF) described in references 7, and 12-16 of the Young paper, is very large and consists of adaptive transversal filtering, classical filtering with parameter estimation, lattice filtering and decision-feedback filtering. ADF provides a wide variety of estimation algorithms with varying response times, but the algorithms that were study by Young and his co-author were limited in the number of interferers that can be rejected by the number of delays (poles and zeroes). Young notes that the advantage of discrete Fourier transform (DFT) based frequency excision is the ability to handle multiple interference sources and the ability to adapt rapidly. The class of DFT algorithms studied by Young are described in references 17-25 of the Young paper. DFT algorithms differ from SAW filter based techniques in that digital technology is used which yields greater freedom in designing the interference removal algorithm and easily provide high dynamic range. The number of interfering signals that can be removed is related to the length of the DFT and may easily extend into the hundreds for a 1024 point DFT and notch depths on the order of 60 dB can be achieved.
However, DFT excision algorithms are software based. Because of this fact, they are too slow for many applications where data rates and traffic volume are very high such as in digital service delivery over cable modem based systems.
In a 1994 paper by Kohri, An Interference Suppressor for CW and Narrow-Band Signals Using Filter Bank on CDMA Communications, published Jul. 4-6, 1994 at the University of Oulu, Oulu, Finland in the proceedings of the IEEE ISSSTA ""94, Kohri proposed a narrowband interference suppressor using a bank polyphase FIR decimating (dividing) filters, a limiter and a bank of combining or interpolator filters at the front end of a CDMA receiver. The transfer function of the filter bank was a series of individual, non-overlapping transfer functions, and no error predicting equalizer was taught to cancel colored noise. Likewise, perfect reconstruction filters were not taught. The limiter was taught as a nonlinear amplifier. The fact that nonoverlapping transfer functions for the filters were used means there will be blind spots in the interference suppressor which could let narrow band interference signals through. The fact that perfect reconstruction filters are not taught, means that the filter banks themselves can introduce distortions in the spread spectrum signal which can cause errors in the payload data.
Therefore, a need has arisen for a narrow band excision algorithm or machine that can deeply notch multiple interfering signals rapidly at the data rates of cable modem systems.
A narrow band interference excision circuit for use in any broadband digital data communication systems such as CDMA or TDMA systems is disclosed herein. xe2x80x9cBroadbandxe2x80x9d as the term is used herein means any transmitted signal with a broad bandwidth such as code division multiplexed signals or TDMA signals where the symbol rate is high. Basically, TDMA signals with a symbol rate approaching or exceeding the chip rate of CDMA systems has as high a bandwidth or higher for the transmitted signals as CDMA signals. The term xe2x80x9ctransmitted signalxe2x80x9d or xe2x80x9ctransmitted signalsxe2x80x9d in the claims is intended to include both TDMA and CDMA signals as well as any other broad bandwidth transmitted signal that could have narrow bandwidth interfering signals therein.
In the preferred embodiment, the excision circuit is comprised of a bank of analysis filters and a bank of synthesis filters separated by an excision circuit. Together, these two collections of filters implement a set of perfect reconstruction filters. The analysis filters function to divide the input signal into a plurality of narrow subbands and have overlapping frequency responses so as to eliminate blind spots in analyzing the entire broadband spectrum. Each narrow subband signal is examined continuously or iteratively to determine if narrowband interference exists in that bin at the time of each iteration. This is done preferably by computing the power of the signals in each subband. If the power in a subband exceeds a threshold, preferably adaptable, then the entire subband is eliminated. This threshold is set so as to detect most instances of narrowband interference. Alternative embodiments take the average power in every bin or do an FFT of every bin to look for noise peaks or compute the variance of the signal amplitude or power at every frequency from the mean and, if any peak exceeds some threshold delta value (which may be programmable or adaptive in some embodiments), the bin is erased or a notch filter is programmed to take out the peak. Since this process is carried out iteratively on every bin, interfering signals which have their bandwidth increase and decrease cause as many bins as they infect at any particular iteration to be erased and to continue to be erased on subsequent iterations until the interference level drops below the threshold delta value for those bins. In other words, the process is an ongoing evaluation of every bin, and every bin that is infected with an interfering signal on any particular iteration will be erased or suppressed.
A bank of polyphase synthesis filters reassembles the composite signal.
Polyphase filters and the Noble Identity are used to enable lowering the complexity of the filter structure by using decimators to lower the sample rate entering each filter and performing interpolation after the synthesis bank to raise the sample rate back up the sample rate going into the decimators. In alternative embodiments, polyphase filters need not be used and more complex analysis and synthesis filter banks are used and more complex detection and cancellation circuits are used so as to be able to work at the higher sample rate.
An equalization circuit with an error predictor comprised of an adaptive FIR filter is coupled to adapt coefficients of the filter and generate a colored noise cancellation signal to remove colored noise from the input to the slicer.